Wearing your defeated foes as clothing is the province of villains, right? Well, not necessarily: EXHIBIT A

How do you resolve that double standard? I mean, doesn't it ever bother you that troll bits become amulets of natural armor, dragon parts become armor, and "create treasure map" does its own creepy thing with your enemies' skin? You'd figure that there ought to be some kind of taboo against cutting up intelligent enemies and using them for spare parts.

The humanoid species wearing Dragon swag doesn't view them as sentient creatures in the sense of fellow humanoids, but as magical beasts of awe, wonder, terror, and everything else in between. Even many of the Outsiders and other Types in the game still are identified as humanoids at a glance. That awe confers status to dem Dragon bits, right or wrong. Were dragonkind to start their own lingerie line, calling it something like Tiamat's Secret, with the fabric/leather being humanoid hair/skin etc, and parading it across the other deities and openly among humanoids, maybe the discussion can be had as to why inherently, this is a bad thing.

Bring it before Abadar in the Plane of Law, and the double standard collapses. On the Material Plane, Mr. Farmer/herder is going to see that Ancient Red and not recognize that its a majestic being that live through eons, collecting the wisdom and knowledge of that time in between. He's going to see something, big & scary, and probably the thing that's been taking from his fields/flocks. He's not going to rationalize with it that this is another being to reason with and coexist alongside. He sees the nearest adventurer -he's flagging them down for some good ole dragonslaying/cookout.

The point it becomes taboo is when there is that acceptance. Most items made of humanoids, and the game is overwhelmingly populated by them and told through their perspectives, are considered evil or taboo.

I've played this concept before in campaigns.

Take a Druid. How does that Druid survive hostile scenarios where endurance is a factor... trapped in demiplanes every now and then? Well... supplies are limited, enemies are not, by nature of what campaigns are. What if the enemies became the supplies?

Enemies as nourishment and equipment soured the Players' on the table, as expected, and it was played for laughs, but I had to take care when I realized that I was crossing peoples' comfort zones for the sake of dark humor. Shaking someone up in real life morally is not worth that cheap giggle... or is it?

The Player playing the Paladin alongside me declared Smite Evil during one of these scenarios. Faced with a population of starving refugees, and with almost no supplies, what was the party to do? Well, me, I always had plenty to share, and they encountered me as they failed their survival roles to forage setting up a soup kitchen... two guesses where the product came from. Pally had to be restrained by the Party. "But I'm feeding the children... won't you feed the needy?"

The argument I had was, why am I confined to Neutral Evil, when my actions and motivations on the whole have been towards greater good, towards the overarching quest, and the welfare of my party? The replies from DM and Players alongside me: its just wrong.

And you know what, I accepted that. They felt that strongly about it, its probably a natural order kind of thing. Its weird, my character felt righteous: survival of the fittest, the food chain isn't hierarchy, its ultimate struggle for existence. Nothing goes to waste, and for one, he's never seen a poop revenant come from a fallen NPC for round 2.

Interesting. I've designed a goblin Promethean alchemist as an NPC in one of my campaigns (somewhat of a henchman to the players) that 'wears' his Homunculus companion as armor, thanks to the construct crafting rules.

I mean, not partaking of 'double standards' would diverge from any normal psychology.
As mentioned, you can engage with or query this, with NPCs or PCs responding to behavior crossing their ethical boundaries.
That's just part of roleplaying. Nothing about this topic prevents just continuing with roleplaying.

It's pretty gross, but I wouldn't say just wearing something made from a sentient creature's skin is inherently evil since they are already dead and don't care. Killing someone sentient specifically to "harvest" their skin or other parts however could be considered evil.

Whether or not wearing sapients is wrong or not has nothing to do with the species in question. The only issue is that the alignment system blue screens when the conclusion is "evil," but that's not a problem with morals, that's just because alignment is trash. As a practical matter, just declare it not-evil and let enlightened people shrug off gnolls wearing skulls and knights wearing dragonskin.

The problem, of course, is that if body part apparel gives you superpowers, somebody is going to hunt people for their parts, so if you make this wear legal, it'll have serious social impact.

Which makes for good aventuring.

PCs could be hired to stop "poachers" -- a.k.a., vicious murderers.

Government might have to regulate all body-part apparel in order to make sure it didn't come from anyone it shouldn't have.

Sapients could use mid-level and high-end magic to grow body parts, sacrifice them, then grow them back as part of an actual industry.

Hard to find a more despicable villain than a merchant that starts a war just to get body parts to sell.

It depends on the creature's moral status. That is, how much value a society attributes to it. Since dragons and trolls have a nasty habit of burning or destroying things people value, it is not hard to kill them or to take their parts as trophies. The same goes for the ruthless slaughter the pcs tend to bring upon bandits.

You could make characters that take issue with wearing the trophies made out of sentient creatures to resolve the double standard but its only a double standard relative to the standard. The sentient critters the pcs chop to bits are most of the time just numbers that try to kill them so they retaliate with their own cold calculus. This could be solved with the gms making sapients surrender and more "sentient." Lore-wise, there seems to be plenty of xeno-phobia when comes to uncivilized or monstrous critters. This can be changed by the gm but most players seem to assume a Lord of the Rings standard for their fantasy.

I'm guessing people don't wear orc faces as trophies because they are too humanoid but then again, orc skulls make fine trophies. There is also the argument that the more civil gear is more effective. Mithral or Adamantine may be more preferable to dragon armor.

My druid purchased and wore gold dragonhide armor with no regrets. The dragon wasn't using it anymore, and it isn't like he hunted and killed the dragon for its hide himself--for all he knew, the dragon died of unrelated causes. He did make a note to eventually find the plane where the dragon's soul ended up and thank them personally. Since the campaign is over, though, I'll have to leave that conversation to the imagination.